In normal types of time-of-flight mass spectrometers (this device is hereinafter referred to as the “TOFMS”), a preset amount of kinetic energy is imparted to ions derived from a sample component to make those ions fly a preset distance in a flight space. The period of time required for their flight is measured, and the mass-to-charge ratio of each ion is calculated from its time of flight. Therefore, if there is a variation in the position of the ions or in the amount of initial energy of the ions at the time when the ions are accelerated and begin to fly, a variation in the time of flight of the ions having the same mass-to-charge ratio occurs, which leads to a deterioration in the mass-resolving power or mass accuracy. As a technique for solving such a problem, an orthogonal acceleration type time-of-flight mass spectrometer, which accelerates ions into the flight space in a direction orthogonal to the incident direction of the ion beam, has been commonly known (hereinafter referred to as the “OA-TOFMS”).
As just described, the OA-TOFMS is configured to accelerate ions in a pulsed fashion in the direction orthogonal to the direction in which a beam of ions derived from a sample component is initially introduced. Such a configuration allows the device to be combined with various types of ion sources which ionize components contained in a continuously introduced sample, such as an atmospheric pressure ion source (e.g. electrospray ion source) or electron ionization source. In recent years, the so-called “Q-TOF mass spectrometer” has also been widely used for structural analyses of compounds or similar purposes. In this device, the OA-TOFMS is combined with a quadrupole mass filter for selecting ions having specific mass-to-charge ratios from ions derived from a sample component as well as a collision cell for dissociating the selected ion by collision-induced dissociation (CID). For example, Non Patent Literature 1 discloses a liquid chromatograph mass spectrometer (hereinafter referred to as the “LC-MS”) for which a Q-TOF mass spectrometer is used as a detector.
The Q-TOF mass spectrometer described above is not only capable of performing an MS/MS analysis but also capable of repeatedly performing a normal mass analysis which does not involve a dissociation operation of ion in a collision cell with high mass resolution. In this case, it is common that a quadrupole mass filter in a previous stage is controlled to function as a type of ion guide that simply transports ions to a latter stage while converging them without performing mass separation to the ions and that the ions are let almost pass through the collision cell without collision-induced dissociation being performed.
In an LC-MS, eluate that contains different components is sequentially introduced into an ion source of the mass spectrometer with the elapse of time. Accordingly, in an LC-MS using a Q-TOF mass spectrometer, ions are repeatedly ejected from the orthogonal accelerator with a predetermined measurement period, and a time-of-flight spectrum with respect to the ejected ions is obtained in the Q-TOF mass spectrometer. In this case, when the measurement period is increased, the measurement time intervals in the Q-TOF mass spectrometer should increase, and there arises a problem that the reproducibility of a peak shape deteriorates when a chromatogram is created based on obtained data, and the quantitative accuracy lowers because the quantitative determination is based on the peak area and the like. For this reason, it is preferable to shorten the measurement period in order to improve the quantitative accuracy.
However, when a normal mass spectrometry is performed with a short measurement period in the Q-TOF mass spectrometer, there is a problem that ions of the next measurement period are ejected from the orthogonal accelerator to the flight space while ions with a long time of flight (that is, ions having large mass-to-charge ratios) are still in the flight space, and thus ions having small mass-to-charge ratios in the next measurement period may catch up with or pass the ions having large mass-to-charge ratios in the previous measurement period, and they may be mixed when reaching the detector.
FIG. 7 at (a) presents an example of time-of-flight spectrum when the measurement period is 200 [μsec] and FIG. 7 at (b) presents the same when the measurement period is 100 [μsec], which is half of it. FIG. 8 at (a) and (b) are enlarged figures of the frame E on the time-of-flight spectrum presented in FIG. 7 at (a) and (b). Most of the peaks observed in the time range of 0 to 15 [μsec] on the time-of-flight spectrum with the measurement period of 100 [μsec] are peaks derived from ions having large mass-to-charge ratios observed in the time range of 100 to 115 [μsec] on the time-of-flight spectrum if the measurement period is taken sufficiently long. Thus, there has been a problem that when the measurement period is shortened, target ions in the previous measurement period appear at positions different from the original positions on the time-of-flight spectrum, which hampers obtaining accurate time-of-flight spectrum.
Patent Literature 1 discloses a technique to find a peak derived from ions in a previous measurement period by comparing a mass spectrum obtained under a different measurement period. Owing to this technique, a peak derived from ions having large mass-to-charge ratios in the previous measurement period can be removed from a time-of-flight spectrum that includes such ions, and enables creating a time-of-flight spectrum on which only a peak derived from the original ions is observed. However, it requires complicated data processing and, further, it is necessary to perform the mass spectrometry twice under different measurement periods to the same sample, and thus it takes time and labor for the measurement.